It is well known to utilize electrical induction heaters to heat thin metal strip to higher temperatures, particularly for annealing purposes. It is also known to heat by induction heating thin strip for the purpose of further reduction of the strip by rolling.
In the past, the frequency of electrical current utilized in conjunction with induction strip heaters has varied from being very high so that the heating is only in the way of skin effect down to 60 Hertz. Such induction heating systems have not proved to be satisfactory for thick slabs in that the induced current only minutely penetrates the thickness of the slab and internal heating must be by way of internal conduction. On the other hand, the exposed surfaces of such a slab is subject to rapid cooling by convection with the result that it is not economically feasible to heat thick slabs utilizing induction heating utilizing a current frequency of 60 Hertz and above.
At the present, 2" inch thick steel slabs are being heated in a gas furnace in an uneconomical process. The difficulty of heating a thick slab utilizing a furnace is that the heating is all done from the outside towards the center of the slab which requires a long period of time for the heat conduction while the heat transferred into the slab is dissipating from the surfaces of the slab.